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  • How come ftp protocol produces transmission errors sometimes if the data is using TCP, which is checksummed?

    - by Cray
    Every once in a while, downloading (especially large) files through ftp will produce errors. I am guessing that's also partly the reason why all major sites are publishing external checksums along with their downloads. How is this possible if ftp goes through TCP, which has checksum inbuilt and resends data if it is transmitted corruptly? One could argue that this is due to the short length of the CRC in the TCP protocol (which is 16bit I think, or something like that), and the collisions are simply happening too often. but 1) for this to be true, not only must there be a CRC collision, but also the random network error must modify both the CRC in the packet, and the packet itself so that the CRC will be valid for the new packet... Even with 16 bitCRC, is that so likely? 2) There are seemingly not many errors in, say, browsing the web which also goes through TCPIP.

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  • Autotesting a network interface

    - by Machado
    Hi All, I'm developing a software component responsible for testing if a network interface has conectivity with the internet. Think of it as the same test the XBOX360 does to inform the user if it's connected with the Live network (just as an example). So far I figured the autotest would run as this: 1) Test the physical network interface (if the cable is conected, has up/downlink, etc...) 2) Test the logical network (has IP address, has DNS, etc...) 3) Connects to the internet (can access google, for example) 4) ??? 5) Profit! (just kidding...) My question relates to step 3: How can I detect, correctly, if my software has connection with the internet ? Is there any fixed IP address to ping ? The problem is that I don't want to rely solely on google.com (or any other well-known address), as those can change in time, and my component will be embbeded on a mobile device, not easy to update. Any suggestions ?

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  • How do I make my multicast program work between computers on different networks?

    - by George
    I made a little chat applet using multicast. It works fine between computers on the same network, but fails if the computers are on different networks. Why is this? import java.io.*; import java.net.*; import java.awt.*; import java.awt.event.*; import javax.swing.*; public class ClientA extends JApplet implements ActionListener, Runnable { JTextField tf; JTextArea ta; MulticastSocket socket; InetAddress group; String name=""; public void start() { try { socket = new MulticastSocket(7777); group = InetAddress.getByName("233.0.0.1"); socket.joinGroup(group); socket.setTimeToLive(255); Thread th = new Thread(this); th.start(); name =JOptionPane.showInputDialog(null,"Please enter your name.","What is your name?",JOptionPane.PLAIN_MESSAGE); tf.grabFocus(); }catch(Exception e) {e.printStackTrace();} } public void init() { JPanel p = new JPanel(new BorderLayout()); ta = new JTextArea(); ta.setEditable(false); ta.setLineWrap(true); JScrollPane sp = new JScrollPane(ta); p.add(sp,BorderLayout.CENTER); JPanel p2 = new JPanel(); tf = new JTextField(30); tf.addActionListener(this); p2.add(tf); JButton b = new JButton("Send"); b.addActionListener(this); p2.add(b); p.add(p2,BorderLayout.SOUTH); add(p); } public void actionPerformed(ActionEvent ae) { String message = name+":"+tf.getText(); tf.setText(""); tf.grabFocus(); byte[] buf = message.getBytes(); DatagramPacket packet = new DatagramPacket(buf,buf.length, group,7777); try { socket.send(packet); } catch(Exception e) {} } public void run() { while(true) { byte[] buf = new byte[256]; String received = ""; DatagramPacket packet = new DatagramPacket(buf, buf.length); try { socket.receive(packet); received = new String(packet.getData()).trim(); } catch(Exception e) {} ta.append(received +"\n"); ta.setCaretPosition(ta.getDocument().getLength()); } } }

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  • Capture DDE Data that is being streamed in to a software

    - by user534391
    Hello, I have a trading software that gets data from the internet. I want to capture that tick data. There is one software that has been made by a local develop which is able to do that and it looks like it uses DDE (NDde.dll, NetSQL.dll). I want to write a custom application that does the same. Any pointers how I can check how the data is being streamed and how to capture that data. I don't think it is encrypted, since the other developer would not have been able to decrypt either. I just need to scan how the software is getting the data. Thank you.

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  • Trouble Extending Network with Apple Airport Extreme (PC won't connect with Ethernet)

    - by 0x783czar
    So I have an Apple Airport Extreme base station that I use to create a wifi network at my house. But on a separate story from this station i have a PC (running Windows 7) that does not have a wireless card. Luckily, I have another Airport Extreme Base Station, so I figured I'd have my second station "extend" the existing network. I asked Apple if this was possible, which it was, and walked through the setup wizard to extend the network. Then I ran an ethernet cable from the Station to my PC. However the PC refuses to connect to the Internet. It says it can access the network, "Unidentified Network (Limited Connectivity)", but that's it. It tells me that my computer does not have an IP address. I tried running the cable to another computer (my Apple MacBook Pro) and got a similar error. Any thoughts?

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  • How get list of local network computers?

    - by KindDragon
    Hello. I am trying to get a list of local network computers. I tried to use NetServerEnum and WNetOpenEnum API, but both API return error code 6118 (ERROR_NO_BROWSER_SERVERS_FOUND). Active Directory in the local network is not used. Most odd Windows Explorer shows all local computers without any problems. Are there other ways to get a list of computers in the LAN?

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  • Can Java ServerSocket and Sockets using ObjectIOStreams lose packets?

    - by Joel Garboden
    I'm using a ServerSocket on my server and Sockets that use ObjectIOStreams to send serializable objects over the network connection. I'm developing an essentially more financial version of monopoly and thus packets being sent and confirmed as sent/received is required. Do I need to implement my own packet loss watcher or is that already taken care of with (Server)Sockets? I'm primarily asking about losing packets during network blips or whatnot, not full connection error. E.g. siblings move a lead plate between my router and computer's wi-fi adapter. http://code.google.com/p/inequity/source/browse/#svn/trunk/src/network Code can be found under network-ClientController and network-Server

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  • Ad-Hoc mode and Cell (how does it work)?

    - by Ori Cohen
    hello, I am setting up a wireless Ad-Hoc network manually in linux using iwconfig and ifconfig. I have gotten everything working, and I think I understand it all except access point. The part that confused me is this: If I have essid='some_id', channel=11, Mode=Ad-Hoc and appropriate routing/ip on both laptops, why do I need to make sure the Cell is the same? I was under the impression that Ad-Hoc worked independently of the access point. I can easily get around this, I just use iwconfig wlan0 ap 00:00:00:00:00:01 Why is this necessary? I've been unable to find a good tutorial on this. If anyone can clear this up I'd appreciate it. Thanks

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  • Does any one know of a packet generator?

    - by Benoit
    We have a networked device, and we would like to perform some tests on how it handles malformed packets. Is there a product out there that can generate arbitrary packets and packet sequences? I would like to be able to specify a set of TCP/IP payloads and it would open a connection and send the data. Obviouly, the TCP/IP checksum should be calculated correctly, etc... Kind of like a wireshark in reverse. Note that I am not interested in network loading and blasting millions of packet.

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  • Windows IIS test server setup

    - by chopps
    hello everyone, I picked up a new server to do some testing and need of a little help in setting up my environment at home. Here is what I would like to do: The test server will be used to test new code and configurations for a SaaS product. I would like from my laptop to enter www.acme.com and have it hit the server. The server is connected to a wireless router. I have windows server 2008 with IIS running on an an IP of 192.168.1.4. What is the best way to set this up? I want to hit the test server for www.acme.com and not go out to the internet. Do i need to mess with the LMHosts file? Thanks for the help. Im sure its easy but have never done this before.

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  • Outgoing UDP sniffer in python?

    - by twneale
    I want to figure out whether my computer is somehow causing a UDP flood that is originating from my network. So that's my underlying problem, and what follows is simply my non-network-person attempt to hypothesize a solution using python. I'm extrapolating from recipe 13.1 ("Passing Messages with Socket Datagrams") from the python cookbook (also here). Would it possible/sensible/not insane to try somehow writing an outgoing UDP proxy in python, so that outgoing packets could be logged before being sent on their merry way? If so, how would one go about it? Based on my quick research, perhaps I could start a server process listening on suspect UDP ports and log anything that gets sent, then forward it on, such as: import socket s =socket.socket(socket.AF_INET, socket.SOCK_DGRAM) s.bind(("", MYPORT)) while True: packet = dict(zip('data', 'addr'), s.recvfrom(1,024)) log.info("Recieved {data} from {addr}.".format(**packet)) But what about doing this for a large number of ports simultaneously? Impractical? Are there drawbacks or other reasons not to bother with this? Is there a better way to solve this problem (please be gentle).

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  • Altruistic network connection bandwidth estimation

    - by datenwolf
    Assume two peers Alice and Bob connected over a IP network. Alice and Bob are exchanging packets of lossy compressed data which are generated and to be consumes in real time (think a VoIP or video chat application). The service is designed to cope with as little bandwidth available, but relies on low latencies. Alice and Bob would mark their connection with an apropriate QoS profile. Alice and Bob want use a variable bitrate compression and would like to consume all of the leftover bandwidth available for the connection between them, but would voluntarily reduce the consumed bitrate depending on the state of the network. However they'd like to retain a stable link, i.e. avoid interruptions in their decoded data stream caused by congestion and the delay until the bandwidth got adjusted. However it is perfectly possible for them to loose a few packets. TL;DR: Alice and Bob want to implement a VoIP protocol from scratch, and are curious about bandwidth and congestion control. What papers and resources do you suggest for Alice and Bob to read? Mainly in the area of bandwidth estimation and congestion control.

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  • orkut via java api

    - by angelina
    Dear all, I m building a web application using servlets/jsp as server side technology. I want connect to orkut server from my web application where user can login,can send as well manage his scraps,get his online friend list.Please share which java api to use

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  • How can i simulate a slow network connection

    - by pilot-riaz
    Hi, I have a Bet Server which sends bet data to a game terminal I would like to simulate a slow network connection. i want the network to be really busy, heavily loaded so i can see how the bet server performs and if it times out when there is a heavy load on the network. Are there any tools i can use to do this? Thanks for any help. Regards, Riaz

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  • UnknownHostException while redirecting queries to google and getting results in JSon object

    - by shilpa
    Loading classifier from D:\PROJECT\classifiers\NERDemo\classifiers\ner-eng-ie.crf-3-all2008.ser.gz ... done [2.0 sec]. Original Query was riot in India. Parsing Queries and expanding tokens from the Ontologies.. {locations=[India], events=[riot]} Search query is null Something went wrong... java.net.UnknownHostException: ajax.googleapis.com at java.net.PlainSocketImpl.connect(Unknown Source) at java.net.SocksSocketImpl.connect(Unknown Source) at java.net.Socket.connect(Unknown Source) at java.net.Socket.connect(Unknown Source) at sun.net.NetworkClient.doConnect(Unknown Source) at sun.net.www.http.HttpClient.openServer(Unknown Source) at sun.net.www.http.HttpClient.openServer(Unknown Source) at sun.net.www.http.HttpClient.<init>(Unknown Source) at sun.net.www.http.HttpClient.New(Unknown Source) at sun.net.www.http.HttpClient.New(Unknown Source) at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(Unknown Source) at sun.net.www.protocol.http.HttpURLConnection.plainConnect(Unknown Source) at sun.net.www.protocol.http.HttpURLConnection.connect(Unknown Source) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source) at org.girs2.SearchHandler.makeQuery(SearchHandler.java:35) at org.girs2.GIRS.search(GIRS.java:37) at org.girs2.GIRS.main(GIRS.java:62) Exception in thread "main" java.lang.NullPointerException at org.girs2.GIRS.search(GIRS.java:44) at org.girs2.GIRS.main(GIRS.java:62)

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  • Java BufferedReader readline blocking?

    - by tgguy
    I want to make an HTTP request and then get the response as sketched here: URLConnection c = new URL("http://foo.com").openConnection(); c.setDoOutput(true); /* write an http request here using a new OutputStreamWriter(c.getOutputStream) */ BufferedReader reader = new BufferedReader(new InputStreamReader(c.getInputStream)); reader.readLine(); But my question is, if the request I send takes a long time before a response is received, what happens in the call reader.readLine() above? Will this process stay running/runnable on the CPU or will it get taken off the CPU and be notified to wake up and run again when there is IO to be read? If it stays on the CPU, what can be done to make it get off and be notified later?

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  • How to model a social news feed on Google App Engine

    - by PEZ
    We want to implement a "News feed" where a user can see messages broadcasted by her friends, sorted with newest message first. But the feed should reflect changes in her friends list. (If she adds new friends, messages from those should be included in the feed, and if she removes friends their messages should not be included.) If we use the pubsub-test example and attach a recipient list to each message this means a lot of manipulation of the message recipients lists when users connect and disconnect friends. We first modeled publish-subscribe "fan out" using conventional RDBMS thinking. It seemed to work at first, but then, since the IN operator works the way it does, we quickly realized we couldn't continue on that path. We found Brett Slatkin's presentation from last years Google I/O and we have now watched it a few times but it isn't clear to us how to do it with "dynamic" recipient lists. What we need are some hints on how to "think" when modeling this.

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  • Can we sniff packets between 2 machines in a network from a third machine using wireshark or etherea

    - by coolcake
    I have a small network in which there are 2 electronic devices and one desktop connected using a switch. Through the desktop with Ethereal/wireshark installed on it, can I sniff the packets that are being communicated between the 2 electronic devices? I cannot install ethereal or wireshark on either of the electronic devices, but need to monitor the traffic between the 2 devices from my desktop, which is connected via the same switch.

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